Tire changing machines are known to comprise, in short, a base with a rotatable platform or equivalent device mounted thereto, both known as a “self-centering unit”, defining a generally horizontal plane, upon which vehicle wheels are laid and locked for tire removal or mounting.
An upwardly facing column is also provided at one end of the base, and supports at its upper end a horizontal operating arm which, in turn, has an adjustable connection device at its free distal end with respect to the column, for connection of a generally known vertical rod, hereinafter referred to as “pole”, which has at its lower end a connector for alternative attachment of various work tools, to be used by tire repairers for removing and mounting tires from and to respective wheel rims, which tools shall be changed according to the various types of tires and rims.
The arm is hinged to the column to swing in a swinging plane, which is also horizontal, like the plane of the self-centering unit, and move the pole parallel to itself from a position of non-interference with the wheel when the latter is locked on the self-centering unit, i.e. away from the self-centering unit, to a work position on the wheel, i.e. converging toward the self-centering unit, and vice versa.
The swinging movements of the arm and the vertical displacements of the pole relative to the arm toward or away from the self-centering unit are manually imparted by the tire changer.
A stop apparatus is also provided between the column and the arm, for stopping the rotation of the operating arm, when the latter is rotated toward the self-centering unit to reach the work position.
This stop apparatus is adjustable for the tire repairer to precisely lock the rotation of the arm in a predetermined position, according to the diameter of a wheel, and for the pole to be substantially aligned with the vertical of a point of the circle in which an active portion of a work took, namely a hook, has to be introduced, particularly between the edge of the rim and the tire bead, for hooking the latter from the inside and extract it from the rim edge, without damaging the rim or the bead and facilitating the tire repairer's work even with tires having particularly rigid sidewalls, like in low-profile tires.
These work tools have hooks rotatably articulated to a shank, integral with the pole, for the hooks to be positioned with a proper orientation using an appropriate pneumatic actuator, which will allow them to be oriented relative to the profiles of the bead and rim edges and to move beyond them as they are introduced therebetween and then to hook the inner edges of the beads during removal thereof from the rims.
The hook-shaped end of the work tool is introduced between the tire bead and the rim edge by a thrust exerted on the tire bead, which progressively overcomes the elastic resistance of the bead, which will bend inwards from the rim, i.e. towards the coupling channel, and open a passage for the work tool end which, after overcoming the inner edge of the bead, rotates about an axis of articulation to the shank and hooks it when the tire repairer reverses the pole displacement direction, i.e. pulls the pole out upwards to overcome the rim edge.
When the hook-shaped tools are forced against a tire bead to bend it and open the passage between the bead and the rim edge, the latter receive a back thrust from the sidewall of the tire which opposes bending.
This back thrust has a generally oblique direction, which extends radial to the tire and is transmitted from the hook to the pole and then from the latter to the operating arm.
One drawback of the prior art is that a radial component of this thrust, i.e. parallel to the plane of the self-centering plane will act upon the pole and hence upon the operating arm which, as a result, will be pushed back and tend to rotate away from the self-centering unit.
Although this rotation is of small amplitude, it is undesired because, if the operating arm, the pole and the work tool mounted to the pole are displaced together, before starting the removal steps they will no longer be at the optimal working position that the tire repairer has defined by operating on the stop apparatus and adjusting it to adapt the position at which the arm rotation will be stopped as a function of the wheel diameter and to precisely establish the point at which the work tool shall be introduced to hook the bead edge and to avoid any damages to the rim or the structure of the sidewall of the tire itself during removal.
Therefore, in the prior art, to obviate this drawback, tire repairers only oppose their own physical force to reaction displacements induced by the introduction of work tools between rim edges and tire beads.
Therefore, the amount of the reaction force opposed by beads will make it difficult, or even impossible, to maintain the optimal position preselected by the stop apparatus.
It should be recalled for clarity that, in a simplified and generally preferred embodiment, a stop apparatus comprises a support bracket, which is fixed in a cantilever fashion to the column and extends therefrom.
A threaded hole extends through the bracket and has a matingly threaded pin transversely engaged therewith, in tightening and loosening directions, and having an operating end facing toward the column and an opposite distal end, which is designed to receive a contacting portion of the operating arm, thereby stopping rotation of the operating arm towards the self-centering unit.
The pin tightening or loosening operation will change the desired position for stopping the rotation of the operating arm towards the self-centering unit and hence will define the precise location at which the work tool will be introduced.